Stretchable interconnects for flexible electronic surfaces

ABSTRACT

A conductive paste and method of manufacturing thereof. The conductive paste comprises conductive particles dispersed in an organic medium, the organic medium comprising: (a) a solvent; and (b) a binder comprising a polyester. The conductive paste typically comprises silver and may contain various other additives. A stretchable conductive layer can be formed by curing the conductive paste.

FIELD OF THE INVENTION

The invention relates to a conductive paste and method of manufacturingthereof. The conductive paste may form a conductive coating, circuitwiring or interconnection capable of being stretched without substantialcrack formation or substantial loss in electrical continuity, and maytherefore find use in capacitive touch technologies and stretchableelectronic surfaces. The solderable version of this paste can be useddirectly in joining two metal surfaces.

BACKGROUND OF THE INVENTION

Capacitive touchscreen displays, for example those present in smartphones, rely on the electrical properties of the human body to detectwhen and where on a display the user is touching. Because of this,capacitive displays can be controlled with very light touches of afinger and generally cannot be used with a mechanical stylus or a glovedhand. For a capacitive device, the capacitive screen is made of aninsulating layer that can also be transparent, such as glass or plastic.If the insulating layer is transparent, then a thin trace of transparentconductive material is used to form electrical patterns on the inside ofthe insulating layer. When a user touches the monitor with his finger,some of the charge is transferred to the user, so the charge on thecapacitive layer decreases. This decrease is measured in circuitslocated at each corner of the monitor. The computer calculates, from therelative differences in charge at each corner, exactly where the touchevent took place and then relays that information to the touch-screendriver software.

Flexible electronics, also known as flexible circuits, result fromassembling electronic circuits by mounting electronic devices onflexible substrates. The flexibility of the circuit is typically limitednot only by the flexibility of the substrate, but also by theflexibility of the electronic devices, circuit lines andinterconnections mounted on the substrate.

Conductive layers, circuit wiring and interconnections can be formedusing a conductive paste, typically comprising conductive particlesdispersed in an organic medium. Existing conductive pastes are notsuitable for selective structuring at the micron-sized level, asrequired, for example, in microelectronics manufacture. In addition,such pastes do not exhibit adequate resistance to environmental effectssuch as, for example, extremes of temperature. Conventional conductivepastes may comprise an epoxy resin. In such pastes, the hardening agentis flaky and hard, a therefore the pastes are not suitable formanufacturing flexible circuits. The electrically conductive pastes arealso prone to form a gel after being kept in storage for a long time.Other conventional polymeric pastes have the disadvantage ofnon-flexibility. As a result, a functional circuit made of conventionalelectrically conductive paste is likely to crack. Conventionalconductive pastes, such as, for example, those containing silvernanoparticles or metal complex particles cannot typically be sintered atlow temperature. Accordingly, when a circuit or conductive layer orinterconnection is formed on a substrate using such conventional pastes,damage to the substrate may occur.

There is a need for a conductive paste capable of forming a flexibleconductive layer, circuit wire and/or interconnection at lowtemperature, which is capable of being stretched without substantialcrack formation or substantial loss in electrical continuity. There isalso a need to make solderable flexible paste for applications such as,for example, decorative LED lighting.

The present invention seeks to tackle at least some of the problemsassociated with the prior art or at least to provide a commerciallyacceptable alternative solution thereto.

The present invention provides a conductive paste comprising conductiveparticles dispersed in an organic medium, the organic medium comprising:

-   -   a solvent; and    -   a binder comprising a polyester.

Each aspect or embodiment as defined herein may be combined with anyother aspect(s) or embodiment(s) unless clearly indicated to thecontrary. In particular, any features indicated as being preferred oradvantageous may be combined with any other feature indicated as beingpreferred or advantageous.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described with reference to the followingnon-limiting Figures, in which:

FIG. 1 shows lines of printed conductive paste according to the presentinvention.

FIG. 2 shows lines of printed conductive paste according to the presentinvention after various lengths of time.

FIG. 3 shows the first and tenth print of a printing process using aconductive paste according to the present invention.

FIG. 4 shows a thermoformed substrate using conductive paste accordingto the present invention.

FIG. 5 shows a thermoformed substrate using conductive paste accordingto the present invention.

FIG. 6 shows a thermoformed substrate using conductive paste accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “polyester” used herein may encompass a polymer that containsthe ester functional group in its main chain.

The inventors have surprisingly found that the conductive paste of thepresent invention may form a conductive coating/layer, circuit wire orinterconnection capable of being stretched (typically up to 150%)without substantial crack formation or substantial loss in electricalcontinuity. The conductive coating or interconnection may also exhibitexcellent volume resistivity, typically of the order of 10⁻⁵ ohm-cm. Inview of these properties, the conductive paste of the present inventionmay be used to fabricate low voltage circuitry, especially on flexiblesubstrates. Accordingly, the conductive paste may be particularlysuitable for use in capacitive touch technologies and stretchableelectronic surfaces.

The conductive paste may be cured at low temperatures at a rapid rate,for example at 65° C. for 20 minutes, and may therefore be particularlysuitable for use by manufacturers with limited curing capacity. Theconductive paste may also be screen printable, stencil printable and/orjettable, and may be capable of continuous printing without flooding.

The conductive paste may exhibit excellent adhesion on substrates suchas, for example, paper-phenolic resin boards, plastic boards (PMMA, PETor the like) and glass-epoxy resin boards. It is also stable at roomtemperature, typically for more than one year.

In comparison to prior art conductive pastes, the conductive paste ofthe present invention may exhibit improved compressive strength,diametral tensile strength and/or flexural strength. The conductivepaste may be capable of being printed in very fine lines, for exampleslines of 50 microns or less in width.

The conductive paste of the present invention may be thermoformableand/or cold drawable, and is suitable for use in 3D printing.

The conductive paste of the present invention may be used to form anelectronic surface. Such electronic surfaces may be conformableskin-like structures with large surface area, high mechanicalflexibility and even stretchability, and multifunctional capabilities.Such structures may enable electro-active surfaces of arbitrary shapewrapped over airplane wings, buildings, robots, three-dimensional (3-D)displays, and human medical prostheses. One of the most difficultchallenges in the development of stretchable electronics is thesimultaneous achievement of both excellent mechanical robustness andelectronic performance. The conductive pastes can be used for a varietyof applications including: LED lighting luminaire fabrication withintegrated thermal and electrical interconnect at very low cost, strainabsorbing interconnects for high thermal cycle fatigue resistance,robotic sensory skins and wearable communication devices andbio-integrated devices. The conductive pates can be used for high CTEmismatch applications such as, for example, direct chip on board heatsink die attach for LED.

Suitable conductive particles include, for example, particles of nickel,copper, carbon, iron, gold, platinum, palladium and mixtures and alloysthereof, as well as conductor coated materials such as organic polymerparticles coated by silver, copper or nickel-silver alloy powder,silver-coated copper powder, silver-coated copper alloy powder,silver-coated copper and/or nickel powder and copper-coated silverpowder. A conductive powder such as, for example, reduced silver powder,may also be used.

The conductive paste may act as a cold solder or a conductive adhesive.The paste forms electrical and thermal interconnects that are superelastic and compliant for a variety of applications (including: LEDlighting luminaire LED assembly, 3-D in mould labelling etc.) withintegrated thermal and electrical interconnect at very low cost.Further, the paste may be used as strain absorbing interconnects forhigh thermal cycle fatigue resistance, robotic sensory skins, wearablecommunication devices and bio-integrated devices. The conductiveadhesives can be used for high CTE mismatch applications such as, forexample, direct chip on board heat sink die attach for LED. Theconductive adhesive may be used for PV ribbon attach to Si cells.

The conductive paste may be overprinted with any solder paste to makesolderable silver paste. Until now it has been difficult to preparesolderable silver paste, since silver can easily leach or dissolve inthe solder. In this approach, copper-coated silver particles aretypically employed. Advantages of the silver paste include: (i) strongjoints and excellent compatibility with different surface finishes, (ii)the proprietary formulation helps avoiding silver dissolution and makesjoints strong, (iii) it is capable of excellent printing with nobleeding or granules, and (iv) components can be mounted easily, with noline breaks or silver dissolution.

The conductive particles preferably comprise silver. Silver may increasethe conductivity of a conductive film formed using the conductive paste.

The conductive particles may be in the form of, for example, spheres,rods, flakes, plates and combinations of two or more thereof.

Preferably, the majority of the conductive particles comprise silverflakes having an aspect ratio (i.e. ratio of length to thickness) offive or less. This may enable fine line printing, for example printlines of about 50 micron or less in width. In this case, typically atleast 90 wt. % of the conductive particles comprise silver flakes havingan aspect ratio of five or less, more typically at least 95 wt. % of theconductive particles, even more typically substantially all of theconductive particles.

Preferably, the majority of the conductive particles have a largestdimension of from 0.5 to 50 microns, preferably from 1 to 20 microns.Such particle sizes may be measured, for example, using a particle sizeanalyser. In this case, typically at least 90 wt. % of the conductiveparticles have a largest dimension in these ranges, more typically atleast 95 wt. % of the conductive particles, even more typicallysubstantially all of the conductive particles. Finer flakes havelimitations in that when an electric field is applied under ahigh-temperature and high-humidity atmosphere, there takes place aphenomenon called migration and resulting electro deposition of silverbetween the wiring conductors and electrodes to cause short circuitingbetween the electrodes or the wires. Moreover, finer particles looseelectrical contact under stretching or bending operation. With biggerflakes it is easy to make paste without use of a three roll mill.

The conductive paste preferably comprises from 30 to 80 wt. % conductiveparticles based on the total weight of the conductive paste, preferablyfrom 55 to 75 wt. %, even more preferably about 65 wt. %. Higher levelsof conductive particles may result in unfavourable rheologicalcharacteristics of the conductive paste and/or reduce the adhesivenessof the conductive paste. Lower levels of conductive particles may resultin unfavourable levels of conductivity.

The solvent is preferably capable of completely dissolving the binder.For high Tg substrates, the solvent preferably may be vaporised from thecomposition below the thermal degradation temperature of the flexiblesubstrate to which the conductive paste is to be applied. Examples ofsuitable solvents include, for example, aromatics, ketones, esters,cellosolves, alcohols, phenols, butyl carbitol, acetates, ethers andcombinations of two or more thereof. The solvent is preferably anon-hydrocarbon polar solvent. In a preferred embodiment, the solventcomprises carbitol acetate.

The solvent preferably has a boiling point of from 150 to 300° C.Solvents having a boiling point below about 150° C. may thicken thecomposition excessively during screening as solvent is evaporatedtherefrom. This may result in plugging of the screens that are used forprinting patterns of the material onto the substrate. Within thislimitation, however, the volatility of the solvent will be selected inconsideration of the method of solvent removal and/or fabrication. Forexample, when the high speed reel-to-reel procedure is used, it isessential that the solvent be removed quite rapidly during processing.Thus lower boiling point solvent must be used, such as those boilingfrom 150° to 175° C. On the other hand, when slower fabricationprocedures are used, less volatile solvents may be used such as thoseboiling from 175 to 300° C., or from 175 to 240° C. In either case thesolvent removal is ordinarily accelerated by mildly heating the printedsubstrate. Typically, the substrate is heated in a hot air oven to 70°to 90° C. when using more volatile solvents in the reel-to-reel process,and 90° to 120° C. when using less volatile solvents in thesemiautomatic processes.

The conductive paste preferably comprises from 5 to 40 wt. % solventbased on the total weight of the conductive paste, more preferably from20 to 30 wt. % solvent. Levels of solvent outside of these ranges mayresult in the conductive paste exhibiting undesirable rheologicalproperties.

Preferably, the ratio of solvent to binder by weight is from 0.15 to0.5. This may provide the paste with particularly favourable viscosity.

The polyester may advantageously be a thermosetting polymer or athermoplastic polymer. The polyester preferably has a thermosettingand/or thermoplasting curing temperature as low as possible, morepreferably less than or equal to 250° C., even more preferably from 30to 100° C. Lower curing temperatures reduce the likelihood of damage toa work piece on which the conductive paste is cured.

The polyester may be linear or branched, and saturated or unsaturated.

The binder may comprise a plurality of polyesters.

The polyester may advantageously exhibit one or more of the followingproperties: a specific gravity of from about 1.0 to about 1.35; aviscosity of from about 800 to about 50000 cPs, elastomeric nature withexcellent film forming characteristics; a softening point (R and B) ofless than 100° C.

The polyester is preferably a copolyester. The term “copolyester” asused herein encompasses a polyester formed from comonomers. Incomparison with conventional polyesters, copolyesters have a reducedtendency to crystallise. In some embodiments, a copolyester may be usedin combination with an amorphous linear polyester. The polyester may bea natural polyester.

The copolyester is preferably saturated, linear and high molecularweight. In a preferred embodiment, the copolymer is formed bycopolymerisation of an aromatic dicarboxylic acid (such as, for example,isophthalic acid or terephthalic acid) and an alkylene glycol (such as,for example, ethylene glycol or propylene glycol). Such copolymers mayprovide particularly enhanced flexibility and stretchability. Aparticularly advantageous copolyester in this regard is poly(ethyleneisophthalate). In another preferred embodiment, the copolymer is anon-crystalline copolyester such as, for example, an ester of adipicacid and neopentyl glycol or an ester of adipic acid, neopentyl glycoland 1,6-hexane diol. Suitable commercial copolymers include, forexample, the Dynapol series of esters, DIC's polyols polyester seriescopolyesters, Primaalloy from Mitsubishi, Vylon from Toyobo chemicals,Setal 173 VS60 and Setal 168 SS80 from Nuplex, Nippon hosei seriespolyester and the like.

The conductive paste preferably comprises from 0.1 to 35 wt. % binderbased on the total weight of the composition, more preferably from 1 to25 wt. % binder. Lower levels of binder may result in undesirable levelsof flexibility and/or stretchability. Higher levels of binder may resultin undesirable rheological characteristics and may reduce the paste'sability to be printerable and/or jettable.

The binder preferably further comprises an elastomer. The presence of anelastomer may increase the stretchability and/or crack resistivity ofthe printed paste. The elastomer preferably comprises an acrylicpolymer. The acrylic polymer may have carboxyl, hydroxyl, or amidegroups, or a mixture of these, and preferably has a weight averagemolecular weight of 25000-500000 and a glass transition temperature offrom −20° C. to +125° C. Acrylic polymers suitable for use in thepresent invention include, for example, alkyl methacrylate, alkylacrylate, hydroxyalkyl acrylate, hydroxyalkyl methacrylate, and may becombined with styrene, acrylic acid or methacrylic acid. Preferredacrylic polymers include an alkyl methacrylate having from 1 to 18carbon atoms in the alkyl group, an alkyl acrylate having from 1 to 18carbon atoms in the alkyl group and a hydroxyalkyl acrylate or ahydroxyalkyl methacrylate each having 2-4 carbon atoms in thehydroxyalkyl group. The conductive paste preferably comprises from 0.1to 20 wt. % elastomer, more preferably from 0.5 to 8 wt. % elastomer.

The binder preferably further comprises an amino resin. The presence ofan amino resin may increase the stretchability and/or flexibility of thepaste. The amino resin may act as a curing agent. The conductive pastepreferably comprises from 0.1 to 20 wt. % amino resin, more preferablyfrom 0.5 to 8 wt. % amino resin.

The conductive paste may further comprise a rheology modifier. Therheology modifier may be used to adjust the consistency and rheology ofthe paste to the particular method of application (e.g. screen printing,stencil printing, jetting). Particularly preferred rheology modifiersinclude, for example, isopropyl myristate (“IPM”), caprylic/caprictriglyceride, ethyl oleate, triethyl citrate, dimethyl phthalate orbenzyl benzoate, cellulose acetate phthalate, ethyl cellulose,hydroxypropylmethyl cellulose, cellulose acetate butyrate or cellulosetriacetate, hydroxyethylcellulose (“HEC”), hydroxypropylcellulose,caboxymethylcellulose, polyethylene glycol or polyvinylpyrrolidone. Evenmore illustrative viscosity modifiers include, but are not limited to,glycerol, glycols, stabilite, alkyl glycidyl ethers, ethyl cellulose,hydroxypropyl cellulose, butyl methacrylate, and feldspar. In thepresent invention the preferred rheology modifier is cellulose ester.Cellulose esters may provide improved hardness, improved silver flakeorientation, high clarity, high gloss, decreased dry-to-touch time,improved flow and leveling, improved re-dissolve resistance, reducedcratering, and reduced blocking. Cellulose esters, particularlybutyrated cellulose esters are alcohol soluble. Such species preferablyhave a butyryl content of about from 45 to 50% by weight and a hydroxylcontent of about from 4 to 5% by weight. Commercial examples of alcoholsoluble cellulose acetate butyrates suitable for use in the presentinvention are available commercially. The rheology modifier may be anatural polyester, This may provide stretchability and thermoformabilityto the silver paste. The conductive paste typically comprises from 0.1to 20 wt. % rheology modifier based on the total weight of theconductive paste, more preferably from 0.5 to 8 wt. % rheology modifierbased on the total weight of the conductive paste.

The conductive paste may further comprises a surface active agent. Thepresence of a surface active agent may increase the flexibility of theconductive paste. Suitable surface active agents for use in the presentinvention include, for example, dipate-based surface active agents,trimellitates, maleates, sebacates, benzoates, epoxidized vegetableoils, sulfonamides, organophosphates, glycols, polyethers and variousethylene oxide-propylene oxide (EO/PO) copolymers,tetrahydrofurfurylalcohol, bis(2-ethylhexyl) phthalate (DEHP),diisononyl phthalate (DINP), bis(n-butyl)phthalate (DnBP, DBP), butylbenzyl phthalate (BBzP), diisodecyl phthalate (DIDP), di-n-octylphthalate (DOP or DnOP), diethyl phthalate (DEP), diisobutyl phthalate(DIBP), di-n-hexyl phthalate, dimethyl adipate (DMAD), monomethyladipate (MMAD), dioctyl adipate (DOA), trimethyl trimellitate (TMTM),tri-(2-ethylhexyl) trimellitate (TEHTM-MG), tri-(n-octyl,n-decyl)trimellitate (ATM), tri-(heptyl,nonyl) trimellitate (LTM), n-octyltrimellitate (OTM), dibutyl maleate (DBM), diisobutyl maleate (DIBM),dibutyl sebacate (DBS), N-ethyl toluene sulfonamide (ortho and 5 paraisomers), N-(2-hydroxypropyl) benzene sulfonamide (HP BSA), N-(n-butyl)benzene sulfonamide (BBSA-NBBS), tricresyl phosphate (TCP), tributylphosphate (TBP), triethylene glycol dihexanoate (3G6, 3GH),tetraethylene glycol diheptanoate (4G7), nitrobenzene, carbon disulfideand P-naphthyl salicylate, triethyl citrate (TEC), acetyl triethylcitrate (ATEC), tributyl citrate (TBC) acetyl tributyl citrate (ATBC),trioctyl citrate (TOC), acetyl trioctyl citrate (ATOC), trihexyl citrate(THC), acetyl trihexyl citrate (ATHC), butyryl trihexyl citrate (BTHC,trihexyl o-butyryl citrate), trimethyl citrate (TMC), nitroglycerine(NG), butanetriol trinitrate (BTTN), metriol trinitrate (METN),diethylene glycol dinitrate (DEGN), bis(2,2-dinitropropyl)formal(BDNPF), bis(2,2-dinitropropyl)acetal (BDNPA), 2,2,2-Trinitroethyl2-nitroxyethyl ether (THEN), sulfonated naphthalene formaldehyde based15 materials, sulfonated melamine formaldehye based materials,polycarboxylic ethers, and dioctyl terephthalate 2,5-dimethyl-2,5hexanediol (DOTP). The paste typically does not contain more than 3 wt.% surface active agent, for example from 0.1 to 3 wt. % surface activeagent.

The paste may further comprises a silane monomer and/or silane oligomer.Such species may improve the wetting characteristics of the conductivepaste and may increase adhesion of the conductive paste to a substrate.The silanes may include non-functional silanes and functionalizedsilanes including amino-functional, epoxy-functional,acrylate-functional and other functional silanes, which are known in theart. Exemplary functionalized silanes includer-glycidoxypropyl-trimethoxysilane, γ-glycidoxypropyltriethoxysilane,glycidoxypropyl-methyldiethoxysilane, glycidoxypropyltrimethoxysilane,glycidoxypropyltriethoxysilane, glycidoxypropylmethyldimethoxysilane,glycidoxypropylmethyldiethoxysilane, 5,6-epoxyhexyltriethoxysilane,epoxycyclohexylethyltrimethoxysilane, and the like. Other exemplaryfunctionalized silanes include trimethoxysilylpropyldiethylene-triamine,N-methylaminopropyltrimethoxysilane,aminoethylaminopropylmethyldimethoxysilane,aminoethylaminopropyltrimethoxysilane, aminopropylmethyldimethoxysilane,aminopropyltrimethoxysilane,aminoethylaminoethylaminopropyl-trimethoxysilane,N-methylamino-propyltrimethoxysilane, methylaminopropyltrimethoxysilane,aminopropylmethyl-diethoxysilane, aminopropyltriethoxysilane,4-aminobutyltriethoxysilane, oligomeric aminoalkylsilane,m-aminophenyltrimethoxysilane, phenylaminopropyltrimethoxysilane,aminoethylaminopropyltriethoxysilane,aminoethylaminoisobutylmethyldmethoxysilane, and the like. Additionalexemplary functional silanes include(3-acryloxypropyl)-trimethoxysilane,gamma-methacryloxypropyltrimethoxysilane,gamma-mercapto-propyltriethoxysilane, and olefinic silanes, such asvinyltrialkoxysilane, vinyltriacetoxysilane, alkylvinyldialkoxysilane,allyltrialkoxysilane, hexenyltrialkoxysilane and the like. Polymersbearing silicon group such as but not limited to poly (methyl siloxane),poly(dimethyl siloxane) and like may be used. BYK307, BYK310, BYK311,hexamethylene disiloxane and the likes are particularly preferredsilanes. If silane monomer and/or silane oligomer is present in theconductive paste, it typically does not exceed 5 wt. %, more typicallyit is present in an amount of from 0.1 to 3 wt. %.

The conductive paste may further comprise a conductive polymer. Thepresence of a conductive polymer may increase the conductivity of theconductive paste. Conductive polymers suitable for use in the presentinvention include, for example, poly(fluorenes), polyphenylenes,polypyrenes, polyazulenes, polynaphthalenes, polyaniline, polypyrrols,polycarbazoles, polyindoles, polyazepines, polythiophenes andpoly(3,4-ethylenedioxy thiphenes) (PEDOT), poly(p-phenylene sulphide)(PSS), copolymer of PEDOT:PSS and the like.

The conductive paste may further comprise an inorganic filler. Inorganicfillers suitable for use in the present invention include, for example,quartz, graphene, graphene oxide, graphite, any suitable silver compoundfrom the group consisting of silver acetate, silver carbonate, silverchlorate, silver chloride, silver lactate, silver nitrate, silverpertafluoropropionate, silver trifluoroacetate, silver trifluoromethanesulfonate, and a mixture thereof.

The conductive polymer and/or inorganic filler may be added directly orin solution. A preferred method is to add in the form of dispersion.Blending with the conductive composition without losing conductivity ofthe composition is critical. The concentration of the conductive polymerand/or inorganic filler therefore, plays critical role. Typically, from1 to 40 wt. % of conductive polymer and/or inorganic filler may beadded, preferably from 5 to 20 wt. %.

In a particularly preferred embodiment:

-   -   the conductive particles comprise silver;    -   the majority of the conductive particles have a largest        dimension of from 0.5 to 50 microns;    -   the conductive paste comprises from 30 to 80 wt. % conductive        particles based on the total weight of the conductive paste;    -   the polyester is poly(ethylene isophthalate); and    -   optionally the conductive paste comprises one or more of an        elastomer, an amino resin, a rheology modifier, a surface active        agent, a silane monomer and/or oligomer, a conductive polymer        and an inorganic filler.

The poly(ethylene isophthalate) is advantageously linear, saturated andhigh molecular weight. The rheology modifier may be a natural polyester,This may provide stretchability and thermoformability to the silverpaste.

In a further aspect, the present invention provides a conductive pastecomprising conductive particles dispersed in an organic medium, theorganic medium comprising:

-   -   a solvent; and    -   a binder comprising one or more of:        -   (a) a branched polyol ester,        -   (a) a saturated or unsaturated polyester,        -   (b) an amino resin,        -   (c) a copolyester derived from one or more glycols and/or            dibasic acids,        -   (d) a polyurethane and/or polyurethane telechelics,        -   (e) an alkyd resin, and        -   (f) cellulose acetate and/or derivatives thereof.

The conductive paste may form a conductive coating capable of beingstretched (typically up to 150%) without substantial crack formation orsubstantial loss in electrical continuity.

In a further aspect, the present invention provides a conductive pastecomprising conductive particles dispersed in an organic medium, theorganic medium comprising:

-   -   a solvent; and    -   a binder comprising a thermosetting polymer and/or thermoplastic        polymer.

The conductive paste may form a conductive coating capable of beingstretched (typically up to 150%) without substantial crack formation orsubstantial loss in electrical continuity.

The conductive particles preferably comprise copper-coated silverparticles, more preferably copper-coated silver flakes. Such particlesmay restrict dissolution and migration of silver. This is particularlyadvantageous when the conductive paste is used as a solderable silverpaste (see above). Typically, the majority of the outer surface of thesilver particles are coated with copper, more typically substantiallyall of the outer surface of the silver particles are coated with copper,even more typically all of the outer surface of the silver particles iscoated with copper.

In an alternative embodiment, the conductive particles may comprisesilver-coated nickel particles and/or silver-coated tin particles. Suchconductive particles may be beneficial when the conductive paste is usedas a solderable silver paste. Typically, the majority of the outersurface of the nickel and/or tin particles are coated with silver, moretypically substantially all of the outer surface of the nickel and/ortin particles are coated with silver, even more typically all of theouter surface of the nickel and/or tin particles is coated with silver.

Preferably, the conductive particles comprises carbon flakes, morepreferably wherein the conductive paste is for overprinting a silverline. Paste made with carbon flakes can be used to overprint on a silverline. Such a paste may restrict silver migration and reduce costs.

The conductive paste may be dispensable to form a line with a resolutionof 50 micron or more.

In a further aspect, the present invention provides a conductiveadhesive or polymer solder comprising the conductive paste describedherein.

In a further aspect, the present invention provides solderable paste forjoining two or more work pieces, the solderable paste comprising:

-   -   the conductive paste described herein; and    -   a solder paste.

In a further aspect the present invention provides a method ofmanufacturing the conductive paste described herein, the methodcomprising:

-   -   providing a solvent, a binder and conductive particles;    -   combining the binder and the solvent to form an organic medium;        and    -   dispersing the conductive particles in the organic medium.

In a further aspect the present invention provides a method of forming astretchable conductive layer on a substrate, the method comprising:

-   -   providing a substrate;    -   applying the conductive paste as described herein to the        substrate; and    -   curing the conductive paste.

The curing may be carried out by, for example, drying. Drying may atleast partially remove solvent from the conductive paste.

The substrate may be a paper-phenolic resin board, a plastic board or aglass epoxy resin board. The substrate may itself be stretchable. Theplastic board may comprise, for example, one or more of PMMA, PET andthe like.

Applying the conductive paste to the substrate preferably comprisesscreen printing, stencil printing and/or jetting. Such techniques mayadvantageously allow automation of the method.

Curing the conductive paste is preferably carried out at a temperatureof up to 250° C., more preferably from 30 to 120° C. Higher temperaturesmay result in damage to the substrate.

In a further aspect, the present invention provides a method of formingan interconnection between two work pieces to be joined, the methodcomprising:

-   -   providing two or more work pieces to be joined;    -   placing the conductive paste as described herein in the vicinity        of the work pieces; and    -   curing the conductive paste.

The preferable features associated with the previous aspect of thepresent invention apply also to this aspect of the present invention.The resulting interconnections may advantageously be flexible. The twoor more work pieces to be joined may be a circuit board and a circuitboard component.

The present invention provides a method of forming a circuit boardcomprising:

-   -   providing a circuit board substrate and the conductive paste        conductive paste described herein;    -   disposing the conductive paste on the circuit board substrate in        a desired circuit pattern; and    -   curing the conductive paste.

The preferable features associated with the previous aspect of thepresent invention apply also to this aspect of the present invention.The circuit board substrate is preferably flexible.

In a further aspect, the present invention provides a method of forminga capacitive touchscreen panel, the method comprising:

-   -   providing a substrate;    -   applying the conductive paste described herein to the substrate;        and    -   curing the conductive paste.

The preferable features associated with the previous aspect of thepresent invention apply also to this aspect of the present invention.The substrate is typically itself flexible. The method may furthercomprise fixing one or more conductors and/or circuits to the substrate,typically at the corners of the substrate.

In a further aspect, the present invention provides a capacitivetouchscreen panel obtainable by the method described herein.

In a further aspect, the present invention provides a mobile telephone,laptop computer or tablet computer comprising the capacitive touchscreenpanel described herein.

In a further aspect, the present invention provides an electronicsurface produced using the conductive paste described herein. Theelectronic surface may be a conformable skin-like structure with largesurface area, high mechanical flexibility and even stretchability, andmultifunctional capabilities. Such structures will enable electro-activesurfaces of arbitrary shape wrapped over airplane wings, buildings,robots, three-dimensional (3-D) displays, and human medical prostheses.

In a further aspect, the present invention provides an airplane wing,building, robot, three-dimensional display, robotic sensory skin,wearable communication device, bio-integrated device or human prosthesiscomprising the electronic surface described herein.

In a further aspect, the present invention provides the use of theconductive paste described herein in a thermoforming method, a colddrawing method, the formation of 3D objects, SMT component mounting,screen printing, jet printing, the printing of lines having a thicknessof 50 microns or more, a method of reducing CTE mismatch (for example,in a direct chip on board heat sink die attach for LED), LED lightingluminaire fabrication, a method of forming strain absorbinginterconnects for high thermal cycle fatigue resistance and multi-stepprinting. When the paste is used in a printing method, it can printlines of 50 micron lines or less. Any shape may be printed, andmulti-step printing is possible. LED lighting luminaire fabrication maybe carried out with integrated thermal and electrical interconnect atvery low cost

The invention will now be described in relation to the followingnon-limiting examples.

Example 1

An organic medium for the composition of the invention was prepared asfollows: to 170 grams of magnetically stirred carbitol acetate (boilingpoint 230° C.) and 40 g downol EPI-1 (from Dow Chemicals) were added 15grams of Dynapol L411 (a linear aromatic polyester resin). Rheologymodifier and adhesion promoter were added to the mixture. The mixturewas heated to 60° C. and stirred at this temperature until a clearsolution was obtained (after approximately 4 hours). The solution wasallowed to cool slowly to ambient temperature.

To the above mixture, 650 parts by weight (65% by weight) of a flakysilver powder with predominant particle size in the range from about 1to 20 microns and an average major particle diameter of 5 μm blended anddispersed uniformly by a mixing and grinding machine to obtain anelectro conductive paste.

The viscosity of the resultant paste after 24 hour ambient temperaturerest measured about 18,000 centipoise (at 20° C.) on a Brookfield RVTviscometer (5 rpm, No. 5 spindle). The resultant paste composition wasprinted on a 100 micron thick electrical grade PET film through 0.5 milsilk screen. The printed parts were dried in a laboratory aircirculating oven at 120° C. for ten minutes to form a conductiveinterconnects. The resulting printed and dried element was tested forresistivity, adhesion (using cellophane tape sold as Scotch brand,#810), scratch resistance and stretching. Resistivity was measured usinga four point probe method and standard multimeter. The conductiveelement was measured for resistivity, then folded in against itself andout against itself so as to create a crease perpendicular to a conductorline. Resistivity was then re-measured. Adhesion was measured with theswitching element on a hard flat surface, and a two inch length ofScotch brand, #810 tape placed over approximately one inch of theprinted conductor pattern. Smoothing pressure was applied with thefingers to insure good bonding of the tape adhesive to the conductiveelement, then peeled back in an upward motion and checked for evidenceof conductor transfer. The fingernail scratch test was performed withthe conductive element on a hard, flat surface. The element wasscratched several times with the fingernail at moderate pressureperpendicular to the printed conductor pattern and checked for evidenceof conductor removal. Stretch test was performed using bench vice(medium bench vice from McMaster). The printed substrate was fixed atboth ends of the vice. The area to be tested was marked. Typicallymarking was 5 cm long. The test area was heated using an air gun at 150°C. and held for 60 seconds. The temperature was selected based on theglass transition temperature of the substrate. Typically stretching wasdone at 10° C. above the glass transition temperature of the substrate.The fixed printed substrate was slowly stretched to approx. 7 cm, i.e.140% of the original length. It was cooled and analyzed under themicroscope at 50× for any cracks. The results observed are set forth inthe following Table 1, which indicate that for the amount of silverpresent, extremely high conductivity (low resistivity) was exhibited,indicating excellent conductivity efficiency provided by the resin. Inaddition, the data of Table 1 indicate that the conductive elementexhibited very good adhesion, flexibility, very good scratch resistance,good pencil hardness and acceptable resistivity after stretching andbending. 3D objects are typically fabricated using cold drawingtechnology. In the laboratory set-up, JT-18 vacuum forming machine wasused to fabricate 3D structures. Plastic film was heated above its glasstransition temperature and thermoformed using rectangular die to createnon-planar structures. These structures were analysed under microscopefor any cracks. Resistivity and all other properties mentioned abovewere tested.

TABLE 1 Test results Dry Resistivity coating Cellophone 140% afterthickness Resistivity tape Pencil Stretching stretching Micron mΩ/sq/miladhesion hardness characteristics mΩ/sq/mil 15 12 Very good 2 H Crackingof 55 line

Example 2

An electro conductive paste was obtained by following the same processas in Example 1 except that following flux composition was used:

Ingredients: Part by weight: Silver flakes 65 Carbitol acetate-solvent18.00 Downol-solvent 4.00 Dynapol L411-binder 1.30 Setal VS60 (trademarkof Nuplex)- 08.60 binder Rheology modifier 2.50 Adhesion promoter 0.60

This composition contained polyester polyol.

Example 3

An electro conductive paste was obtained by following the same processas in Example 1 except that following flux composition was used:

Ag flakes 65.00 Carbitol acetate 18.90 Thinner 3.2 Dynapol 411 2.60Setal_US_136_BB_157 7.20 (Trademark of Nuplex) Rheology modifier 3.10

This composition contained branched polyester and polyamine curing agent

Example 4

An electro conductive paste was obtained by following the same processas in Example 1 except that following flux composition was used:

Ingredients: Part by weight: Carbitol acetate 17.0 Thinner 4.0 DynapolL411 1.5 Setal VS80 (trademark of Nuplex) 5.7 Setal_US_136_BB_157(Trademark of 1.4 Nuplex) Rheology modifier 5.40

This composition contained branched polyester polyol and elastomer forbetter stretchability.

Example 5

An electro conductive paste was obtained by following the same processas in Example 1 except that following flux composition was used:

Ingredients: Part by weight: Butyl carbitol 17.0 Az NPG/HD 4.0 DynapolL411 1.5 Setal VS80 (trademark of Nuplex) 5.7 PMMA 1.1 Rheology modifier2.70 BYK4510 3.0

This composition contained copolyester resin, branched polyester polyoland elastomer for better stretchability.

Example 6

An electroconductive paste was obtained by following the same process asin Example 1 except that following flux composition was used:

Ingredients: Part by weight: Carbitol acetate 18.0 Thinner 4.0 DynapolL411 1.5 Setal VS80 (trademark of Nuplex) 5.7 PMMA (poly (methylmethacrylate) 1.1 Rheology modifier 3.0

This composition contained branched polyester polyol and elastomer forbetter stretchability. Silver flakes content is increased.

Example 7

An electroconductive paste was obtained by following the same process asin Example 1 except that following flux composition was used:

Ingredients: Part by weight: Carbitol acetate 17.0 Thinner 4.0Copolyester (DIC chemicals product 1.5 no. OD-X-2560)Setal_US_136_BB_157 (Trademark of 5.7 Nuplex) PMMA 1.1 Conductive filler1.0 Rheology modifier 4.70

This composition contained conductive filler, branched polyester polyoland elastomer for better stretchability and conductivity.

Example 8

An electro conductive paste was obtained by following the same processas in Example 1 except that following flux composition was used:

Ingredients: Part by weight: Carbitol acetate 17.0 Thinner 4.4 DynapolL411 1.5 Setal_US_136_BB_157 (Trademark of 5.7 Nuplex) Graphite 1.0Rheology modifier 5.40

This composition helps reduce silver migration.

Example 9

An electro conductive paste was obtained by following the same processas in Example 1 except that following flux composition was used:

Ingredients: Part by weight: Carbitol acetate 17.0 Thinner 4.4 DynapolL411 1.5 Setal_US_136_BB_157 (Trademark of 5.7 Nuplex) Graphene 1.0Rheology modifier 5.40

This composition uses filler for better electrical continuity whenstretched and thermoformed.

All compositions in Examples 2-9 were prepared using 65 parts by weightof silver flakes as prepared in accordance with Example 1. The pastecompositions were screen printed and dried as discussed in Example 1 andproperties tested. The results, which are summarized in Table 2,indicate that the best overall performance was obtained with a medium inwhich the resin was 25% of the total weight of the medium. Graphite andgraphene addition significantly enhances tensile properties of theconductive printed lines.

TABLE 2 Summary Test Results % Resistivity Dry after coating Cellophane140 stretching/ thickness Resistivity tape Pencil Stretching formingExamples Micron mΩ/sq/mil adhesion hardness characteristicsThermoforming mΩ/sq/mil 2 16 12 V good 2 H No cracks Possible  55 3 1612 V good 2 H No cracks Possible  55 4 16 15 V good 2 H No cracksPossible  53 5 16 29 V good 2 H No cracks Possible  60 6 16 30 V good 2H No cracks Possible  85 7 16 30 V good 2 H No cracks Possible  85 8 1732 V good 2 H No cracks Possible  95 9 17 34 V good 2 H No cracksPossible 110

Example 10—Printing Evaluation

A thorough printing evaluation was conducted on the pastes. A DEK03xiautomated printer was used for the evaluation. Other parameters used aregiven in Table 3.

TABLE 3 Printing parameters Test vehicle PET film Stencil Mesh screenStencil thickness <1 mil Aperture size 0.5 mm × 152 mm and 1 mm × 152 mm

The results of the study are as follows:

Print window optimization: print window optimization was carried out toidentify the optimized printing window to achieve a defect freeprinting. A mesh screen with less than 1 mil thickness was used forthis. The study was also conducted for conventional and other pastesamples. Dense lines of printed paste of Example 8 are shown in FIG. 1.The print window for this paste was optimized at: Print Speed—200mm/Sec, Pressure—15 Kg, Snap Off Speed—0.5 mm/Sec, Flood height: 3.5 mm,Printing process: Print and Flood. The parameters show that the paste ofExample 8 is able to print at high speed thus giving high throughput. Itwas observed that the paste does not compact post curing, thus apertureheight is a vital factor to be considered for paste deposit thickness.The reason for the non-compactness of the paste is that it containsmicron size flakes and, unlike nano-sized silver powder, there is nosintering phenomenon or grain boundary diffusion.

Response to pause: PET films were printed with paste fresh out of thejar (paste not kneaded). This provides a means of quantifying “out ofjar performance”. The test seeks to capture the initial performance byinspecting print deposits. The response to pause test providesinformation on how the paste responds to a pause in printing atdifferent pause intervals. All example pastes yielded good response topause test. In all cases, no bridges or bleeding was observed. Printdefinition was uniform and dense in all cases. This can be seen, forexample, in FIG. 2, which shows printing images of the paste of Example2 at t=0 (left hand side), t=60 minutes (middle) and t=120 minutes(right hand side).

Continuous Print: This test was performed to determine the drying effectof paste in the aperture after every print, without flooding the pasteon stencil. The test is important as it gives direct indication ofthroughput and material loss. Pastes in this invention were able to give10 prints without flooding. All conventional commercial pastes severelyfailed in this application. The texture of the pastes of the presentinvention was smooth and the printed structures do not have anyporosity. Without being bound by theory, it is considered that this isdue to the solvent/binder of the pastes of the present invention. Thepossibility of continuous printing is also due to the novel solvent andrheology modifiers incorporated into the pastes. FIG. 3 shows theprinting performance of the paste of Example 5. Dense lines with nodefects were observed even after the 10^(th) print. The paste requiresno flooding which is unique. All other commercial pastes cannot beprinted without flooding.

Stencil life: In order to determine the stencil life of the paste, itwas kneaded continuously at 25° C. Brookfield viscosity measurementswere taken every hour including T-0. The sample was also printed at TOand at the end of the test in order to quantify printability and totrack any changes in the paste's behavior over a period. This test givesan idea of how the paste performs on the stencil when kept open for along time or run continuously for a longer time. All example pastes inthis disclosure showed excellent performance in this test. They have astencil life for four hours or more. No bubbling or abnormality inprinting observed. The printed paste has a smooth structure on thesubstrate and a stencil life up to four hours. All pastes performed wellthroughout four hours of stencil life. No sticking to squeegee wasobserved. Good rolling was observed throughout four hours of kneading

All example pastes performed well in all printing tests. Conventionalpaste was inferior in continuous printing. Table 4 gives a summary ofthe results of the printing study.

TABLE 4 Comparison of printing performance of all pastes Properties ofpaste Tests composition in this invention Print window optimization MeshScreen Speed : 200 mm/sec Pressure: 13.5 kg Print and Flood Response topause Upton 2 hr No. of prints without flooding 10 (Wipe frequency)Ambient Viscosity @ 0.5 RPM-450% increase Temperature Behavior @ 5.0RPM-396% increase Stencil Life Test Stencil Upton 4 hrs life

Example 11—Thermoforming

Thermoforming is a process in which a flat thermoplastic sheet is heatedand deformed into the desired shape. Heating is usually accomplished byradiant electric heaters, located on one or both sides of the startingplastic sheet at a distance of roughly 125 mm (5 in.). The duration ofthe heating cycle needed to sufficiently soften the sheet depends on thepolymer, its thickness and colour. The methods by which the forming stepis accomplished can be classified into three basic categories: (1)vacuum thermoforming, (2) pressure thermoforming, and (3) mechanicalthermoforming. In this investigation, vacuum thermoforming was used.Printed lines were thermoformed using specific mould designs. Linecontinuity, thermoformed object and overall performance of the paste wasinvestigated.

Once printed, the substrate may undergo 3D deformation and the pasteshould be able to retain its conductivity and other physical propertieswithout getting delaminated. The printed substrate may undergo colddrawing, thermoforming and similar 3D deformation activity in order toproduce e.g. 3D components for stretchable electronic surfaces. Theimportant objective of this work was to form a paste which shouldwithstand such operations without losing physical properties such asconductivity or adhesion or getting lines cracked. In this studygenerally pastes were cured at 650° C. for 10 minutes in art aircirculating oven. A variety of substrates were used for this study.Forming requires specific substrates for specific applications. WhilePET is common in industry, PMMA (Polymethyl methacrylate) and PC(Polycarbonate) are also sometimes used. All three substrates were usedin this evaluation (see Table 5).

TABLE 5 Various substrates and their properties for vacuum forming.Thickness Tg Ease of Base Material Source (μm) (° C.) Forming PET McMaster, USA 100 86 V. Difficult APET Spearepet, India 200 80 Easy(Vacuum Formable Grade) Monocrystalline Furukawa Electric, 1000 78 V.Difficult PET Japan PMMA Nudec, Spain 1500 109 Easy Polycarbonate (PC)Flexi Tuff, India 1000 147 Easy (Non UV coated)

As stated above, PET is widely used substrate. This substratethermoforms at around 120-125° C. Various molds were custom made for thethermoforming study. All example paste compositions showed excellentresults on the PET substrate. FIGS. 4-6 show thermoformed substratesusing the paste of Example 5. As can be revealed from microscopicimages, no cracks were observed. The line was electrically continuouswith a resistance of 55 ohm/sq. As shown in FIG. 5, lines could beprinted with a thickness of 100 microns. As shown in FIG. 6, it waspossible to achieve both 90° and 180° bends.

Example 12

Silver paste was prepared per example one, with the only differencebeing that copper coated silver powder was used as a starting point. Thepaste was dispensed on a plastic surface. It was dried and Alpha's lowtemperature solder paste, ALPHA® CVP-520 (tin bismuth) paste wasoverprinted. On this assembly, SMT components were mounted and reflowedusing standard reflow profile. The joints were inspected for voids andshear strength.

Similarly, using special stencils, LED assembly was mounted on this Agpaste/solder paste combination. In both examples, it was found thatstretchable silver paste was capable of forming excellent solder jointsand lighting LEDs with acceptable die shear strength.

Example 13

Silver paste was prepared per example one, with the only differencebeing that silver coated tin powder was used as a starting point.Similar results to that in Example 12 were obtained.

Example 14—Ag Paste as Conductive Adhesive (“Compliant” Paste)

Paste prepared similar to example one was dispensed using a manualpneumatic needle dispenser. The paste was dried at 65° C. One more layerof silver paste was overprinted on the cured Compliant trace. LEDs wereassembled on this conductive traces and the entire assembly was cured at120° C. in a convection oven for 30 minutes. No conductive adhesive wasused.

LED glow was successfully observed and the measured die shear strengthwas 5 Kg. This confirms that compliant paste can act as a conductiveadhesive and a polymer solder.

Example 15

Paste was prepared like example one, except for the fact that carbon(graphite) flakes were used as a starting point. The paste shows best inclass stretchability and can be overprinted on silver traces to avoidsilver migration and reduce cost.

The foregoing detailed description has been provided by way ofexplanation and illustration, and is not intended to limit the scope ofthe appended claims. Many variations in the presently preferredembodiments illustrated herein will be apparent to one of ordinary skillin the art and remain within the scope of the appended claims and theirequivalents.

The invention claimed is:
 1. A conductive paste for joining two or morework pieces, the conductive paste comprising: a conductive pastecomprising conductive particles dispersed in an organic medium, theorganic medium comprising: a solvent; and a binder comprising anelastomer, wherein the elastomer is formed of a copolyester selectedfrom the group consisting of an ester of adipic acid and neopentylglycol, an ester of adipic acid, neopentyl glycol, and 1,6-hexanediol, acopolyester formed by the copolymerization of an aromatic dicarboxylicacid and ethylene glycol, a copolyester formed of an aromaticdicarboxylic acid and propylene glycol, and combinations of theforegoing; wherein the conductive particles comprise silver-coatedcopper particles.
 2. The conductive paste of claim 1, wherein the binderfurther comprises one or more of: (a) a branched polyol ester, (a) asaturated or unsaturated polyester, (b) an amino resin, (c) acopolyester derived from one or more glycols and/or dibasic acids, (d) apolyurethane and/or polyurethane telechelics, (e) an alkyd resin, and(f) cellulose acetate and/or derivatives thereof.
 3. The conductivepaste of claim 1, wherein the binder further comprises at least one of acopolyester, and a polyester polyol.
 4. The conductive paste of claim 1,wherein the solvent is selected from the group consisting of aromatics,ketones, esters, cellosolves, alcohols, phenols, butyl carbitol,acetates, ethers and combinations thereof.
 5. The conductive paste ofclaim 1, wherein the elastomer further comprises an acrylic polymerobtained from monomers selected from the group consisting of alkylmethacrylate, alkyl acrylate, hydroxyalkyl acrylate, and hydroxyalkylmethacrylate.
 6. The conductive paste of claim 1, wherein the organicmedium further comprises a copolymer formed from a copolymer selectedfrom the group consisting of of ethylene glycol, propylene glycol, anester of adipic acid and neopentyl glycol, an ester of adipic acid,neopentyl glycol, 1,6-hexane diol, and combinations of one or more ofthe foregoing.
 7. The conductive paste of claim 1, wherein theconductive paste is curable at a temperature between 30 and 120° C. 8.The conductive paste of claim 1, wherein the conductive paste remainsstable at room temperature for at least one year.
 9. The conductivepaste of claim 1, wherein the conductive paste is capable of being atleast one of a stretchable conductive adhesive and a polymer solder. 10.The conductive paste of claim 1, wherein the conductive paste is atleast one of thermoformable, cold drawable, and 3D printable.
 11. Theconductive paste of claim 1, wherein the conductive paste is dispensableto form lines of 50 microns or less in width.
 12. The conductive pasteof claim 1, wherein the conductive paste is capable of being applied asa conductive coating that is capable of being stretched up to 140%without substantial crack formation or substantial loss in electricalcontinuity.
 13. The conductive paste of claim 1, wherein the conductivepaste comprises silver particles with a predominant particle size in therange from about 1 to 20 microns and an average major particle diameterof 5 μm.
 14. A conductive coating comprising the conductive paste ofclaim 1, wherein the conductive coating is capable of stretching up to140% without substantial crack formation or substantial loss inelectrical continuity.
 15. The conductive coating of claim 14, whereinthe conductive coating is capable of being stretched up to 150% withoutsubstantial crack formation or substantial loss in electricalcontinuity.